mongodb: add patch for clang failure.

Closes Homebrew/homebrew#22696.

Formula/mongodb.rb

@@ -19,6 +19,11 @@ class Mongodb < Formula
     sha1 'b4d7e33054b9daef2504bcdb8f26ef43dbea6aaf' => :snow_leopard
   end
 
+  def patches
+    # Fix Clang v8 build failure.
+    'https://github.com/mongodb/mongo/commit/be4bc7.patch'
+  end
+
   depends_on 'scons' => :build
   depends_on 'openssl' => :optional